A battery, which is an example of an electric storage device, requires pouring of an electrolyte solution into a case of the battery in a manufacturing process of the battery. For this purpose, an electrolyte solution pouring opening is formed at the case of the battery in many cases and the electrolyte solution is poured through the electrolyte solution pouring opening.
In this case, the electrolyte solution is poured into the case through the electrolyte solution pouring opening and the electrolyte solution pouring opening needs to be sealed after a predetermined step. As widely used methods of sealing, the electrolyte solution pouring opening is sealed by screwing an electrolyte solution plug thereinto, or the electrolyte solution pouring opening is sealed by being covered with a lid-shaped member and welding the member as described in Patent Document 1 listed below.